The present invention relates in general to coupling electronic signals into and out of an integrated circuit (IC). More specifically, the present invention relates to systems, fabrication methodologies and resulting bump metallization structures which provide superconducting electrical coupling and secure mechanical adhesion between a bump and the metallization structure.
Semiconductor devices are used in a variety of electronic and electro-optical applications. ICs are typically formed from various circuit configurations of semiconductor devices formed on semiconductor wafers. Alternatively, semiconductor devices can be formed as monolithic devices, e.g., discrete devices. Semiconductor devices are formed on semiconductor wafers by depositing many types of thin films of material over the semiconductor wafers, patterning the thin films, doping selective regions of the semiconductor wafers, etc.
In a conventional semiconductor fabrication process, a large number of semiconductor devices are fabricated in a single wafer. CMOS (complementary metal-oxide semiconductor) is the semiconductor fabrication technology used to form the transistors that are manufactured into most of today's computer microchips. In CMOS technology, both n-type and p-type transistors are used in a complementary way to form a current gate that forms an effective means of electrical control. Processing operations performed later in CMOS technology fabrication sequences are referred to as back-end-of-line (BEOL) CMOS processing, and processing steps performed earlier in CMOS technology fabrication sequences are referred to as front-end-of-line (FEOL) CMOS processing.
After completion of device level and interconnect level fabrication processes, the semiconductor devices on the wafer are separated into micro-chips (i.e., chips), and the final product is packaged. IC (or chip) packaging typically involves encasing the silicon chip(s) inside a hermetically sealed plastic, metal or ceramic package that prevents the chip(s) from being damaged by exposure to dust, moisture or contact with other objects. IC packaging also allows easier connections to a PCB. The purpose of a PCB is to connect ICs and discreet components together to form larger operational circuits. Other parts that can be mounted to the PCB include card sockets, microwave connectors, and the like.
Wire bonding is a known BEOL operation for forming electrical interconnections between a PCB and other components (e.g., external components, card sockets, microwave connectors, chip carriers, etc.). In wire bonding, a length of small diameter soft metal wire (e.g., gold (Au), copper (Cu), silver (Ag), aluminum (Al), and the like) is attached or bonded without the use of solder to a compatible metallic surface or pad mounted on a PCB. The actual bond between the wire and the pad can be formed in a variety of ways, including the use of thermo-compression, thermo-sonic and ultrasonic techniques. Although wire bonding is widely used, the additional wire bond hardware, particularly in microwave/radio frequency (RF) applications, is manually intensive to fabricate, suffers from low temperature CTE (coefficient of thermal expansion) mismatches, is difficult to reliably repeat, causes signal path problems, increases cost, adds bulk and introduces extraneous microwave cavity modes.
So called “flip chip” assembly methodologies provide an alternative to wire bonding. Flip Chip assembly is the direct electrical connection of face-down (i.e., flipped) electronic die onto organic or ceramic circuit boards by means of conductive bumps on the chip bond pads, which are also known as “under bump metallization” (UBM). The conductive bumps can be formed as small spheres of solder (i.e., solder balls), which are bonded to contact areas or pads of semiconductor devices. A conventional flip chip assembly methodology can include placing solder material on a semiconductor chip/substrate, flipping the chip over, aligning the solder with the contact pads (i.e., UBM) on the chip, and re-flowing the solder in a furnace to form the solder into spherical shapes and establish the bonding between the solder bumps, the UBM and the chip. Flip chip assembly methodologies can provide electrical connections with minute parasitic inductances and capacitances. In addition, the contact pads (i.e., UBMs) are distributed over the entire chip surface rather than being confined to the periphery, as in wire bonding. As a result, the chip area is used more efficiently, the maximum number of interconnects is increased, and signal interconnections are shortened. Accordingly, flip chip assembly methodologies have advantages over traditional face-up wire bonding techniques including real-estate utilization, performance, reliability, and cost.
For chip configurations in which bump bonds carry electronic data and other signals into and out of a chip, it can be desirable to minimize signal loss from the electronic data and other signals that are carried across bump bonds. One method of minimizing such signal loss is to configure the bump/UBM interface to be fully superconductive. In general, superconductivity is the ability of certain materials to conduct electric current with practically zero resistance.
In many cases, the bump metal of choice is indium or indium alloys, with the material deposited in various ways onto a compatible UBM film stack. Because the thick indium-based bump metal is deposited in a separate process from the UBM films, certain constraints are placed on the composition of the UBM film. In particular, the top surface of the UBM should not be oxidized during indium deposition/plating in order to form a good bond between the indium alloy and the UBM. As a result, it can be desirable to use materials which form strong adhesion to the UBM stack. Materials which form such a mechanical bond may well not form a superconducting connection (and in fact may not even provide an electrical contact). For example, noble metal on the UBM films can provide superior mechanical/metallic connections because little to no oxide is present on the noble metal surface, while possibly not providing a superconducting contact. Noble metals are particularly attractive to provide a mechanical bond because the level of oxidation on the surface of noble metals, even in a moist environment, is very low.